In the medical field, various types of body implantable leads are known and used. One type of commonly used implantable lead is an endocardial pacing lead.
Endocardial pacing leads are attached at their proximal end to an implantable pulse generator and at their distal end to the endocardium of a cardiac chamber. The distal end of an endocardial lead may engage the endocardium by either an active fixation mechanism or a passive fixation mechanism.
Passive fixation mechanisms, such as a tine assembly, lodge or passively fix the lead to the heart. Active fixation mechanisms use a structure, such as a helix or hook, to engage into or actively fix themselves onto the heart.
A preferred means for introducing an endocardial lead into the heart is through a vein. Specifically, such a lead, called a transvenous lead, is introduced into and maneuvered through the vein so the distal end is positioned within the heart. Introduction of an active fixation lead, however, in such a manner presents difficulties. In particular, an exposed sharpened helix, may damage a vein during introduction. Thus many active fixation leads have helixes which either retract into the lead body or are shielded during introduction. See, for example, U.S. Pat. No. 4,972,848 of the Di Domenico (helix shielded within lead body which may be extended to engage cardiac tissue): U.S. Pat. No. 5,003,992 of Holleman et. al (plunger through helix guards against damage to tissue by the helix, plunger may be retracted to permit helix to engage tissue) and U.S. Pat. No. 4,827,940 of Mayer et. al. (soluble cover shields helix until positioned approximate fixation site.)
Retraction into and extension from the lead body is a preferred method of shielding the helix in a transvenous, endocardial lead. Various means may be used to achieve such retraction and extension. One means used has been with a stylet. For example, U.S. Pat. No. 4,217,913 to Dutcher et al. discloses a lead having a coiled conductor encased within an insulating material. A ridged helix is attached at the distal end of the lead by a piston. The lead body is shaped to permit the introduction of a stylet therethrough. The stylet and piston, in particular, are shaped to mutually engage one another. Specifically, the stylet has a screwdriver shaped distal tip which engages into a slot in the head of the piston. When these two objects are mated together and the stylet is rotated, it transmits rotational torque to the piston. The piston, in turn, rotates the attached helix such that when rotated in the first direction, the helix is advanced out of the distal end of the lead body so it may introduced into endocardial tissue. When rotated in a second direction the helix retracts into the lead body so it disengages endocardial tissue. In various endocardial screw-in lead designs the helix may be either electrically isolated from a spaced electrode or may itself constitute the electrode.
One difficulty sometimes encountered with such a design is when it becomes necessary to bend the lead so the distal end may be positioned in a desired location. This type of bending is often required, for example, when the lead needs to be implanted in the right atrium of the heart. Bending of the lead body may be readily accomplished by various means. Inducing a torque to the distal end while bent, however, presents a challenge. Specifically, because the lead is bent rotation of the proximal end of the stylet will tend to cause the bent stylet and thus bent lead body to rotate thereabout, that is, it will cause the entire bent lead to rotate such that the distal end dislodges from its desired location. It is preferred if rotation of the stylet, even though bent, does not cause the distal end to dislodge, but rather only rotates about its longitudinal axis.
One solution which has been proposed to permit a helix to be rotated at the distal end of a lead by a stylet while the lead and stylet are bent may be seen in the U.S. Pat. No. 4,350,169 to Dutcher et al. As seen, this discloses a stylet having a narrowed or necked region adjacent to the distal end of the stylet. This necked or narrowed section increases the flexibility of the stylet in that region without detracting from the ability of the stylet to transmit torque from its proximal end to its distal end. In particular it was believed that by providing a readily flexible necked or narrowed intermediate section the stylet would bend in conformity with the lead body but would rotate about its longitudinal axis when rotated.
In practice, it has been found, however, that this necked or narrowed section in the stylet still imparts a considerable tendency to straighten the bend or curve in the lead and thus dislodge the tip of the lead from the desired position. In order to offset this straightening tendency, the J-shape of the preformed atrial leads must be made inherently stiffer. This stiffening can cause additional chronic trauma at the implant site which can lead to higher stimulation thresholds or poor sensing or both during chronic use of the lead. These difficulties have slowed wide-spread adoption of transvenous endocardial screw-in leads.
One device used to provide the transmission of torque about a bend is disclosed in U.S. Pat. No. 5,165,421 of Fleischhacker et al. which discloses a hollow lumen cable apparatus. Specifically this apparatus comprises a pair of counter wound coils. These coils permit the apparatus to be highly flexible and pliant while permitting torque to be readily transferred. One drawback with such an apparatus, however, is its flexibility. For lead implantation, although it may be necessary for the lead to bend, it is still necessary for the stylet to provide some degree of stiffness to the lead.